Genetic variability was high among 135 isolates of Escherichia coli isolated from quarters of dairy cows with mastitis in two New York state Holstein dairy herds. Identical patterns of pulsed field gel electrophoresis (PFGE) results were only seen in isolates from the same quarter of the same cow at different stages of lactation. This underlines that E. coli are very rarely spread from cow to cow during milking time but are rather derived from the environment. It also shows that some E. coli strains are able to persist in the mammary gland for longer periods leading to chronic infection with recurrent clinical episodes. PFGE patterns of the isolates were furthermore independent of the presence of genes encoding for antimicrobial resistance (AMR).

Nearly all isolates included in this study that were tested were resistant against ampicillin, aztreonam and cefaclor (90%) and many were resistant against streptomycin, nitrofurantoin, sulfisoxaole, tetracycline, cefuroxime, fosfomycin, gentamicin and cephalotin (15 to 40 %). Fewer (<15%) isolates were resistant against amikacin, fluoroquinolones, florfenicol and 3rd and 4th generation cephalosporins. Interestingly, the presence of genes encoding for AMR and expression of AMR in vitro were not fully congruent. Some isolates displayed AMR without carrying known genes for this resistance while others carried the genes but were susceptible in vitro.

If antimicrobial treatment of coliform mastitis is considered an option it has to commence before the results of antimicrobial susceptibility testing are available because of the peracute course of the disease. The study underlines that AMR of E. coli cannot be deduced from earlier cases of clinical mastitis in other cows but has to be determined retrospectively in any new infection.

Knowledge on resistance patterns observed in the past may,however, guide the treatment decision. The use of antimicrobials that have regularly been shown to be ineffective in vitro in earlier cases of mastitis should be avoided. Ideally this knowledge should be available on farm. As a second choice regional data may be used. While data derived from two farms as in this study are not helpful to guide treatment decisions their analysis is helpful to improve the understanding of the variability of AMR in E. coli associated with mastitis.